diff --git a/oft b/oft
index 439bae8..3e26d52 100755
--- a/oft
+++ b/oft
@@ -124,6 +124,11 @@
     # Running from source tree
     sys.path.insert(0, pydir)
 
+try:
+    import oftest.message
+except:
+    sys.exit("Missing OpenFlow message classes: please run \"make -C tools/munger\"")
+
 import oftest.testutils
 import oftest.ofutils